WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is typically connected to the ring with optical connectors. The optical add/drop element permits both addition and extraction of channels to and from the ring. A particular node that allows the addition and extraction of all the channels is commonly referred to as a hub or central office node, and typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring.
FIG. 1 shows a functional block diagram of a conventional WDM ring network 100. Ring network 100 includes a plurality of nodes 102-105 connected along a continuous, or looped, optical path 110. Each of these nodes is typically linked by a segment of optical fiber.
FIG. 2 shows an exemplary node 200 in more detail. Typically nodes 102-108 have a construction similar to node 200. Node 200 generally includes an optical add/drop multiplexer (OADM), user interfaces, and a network management element. In the case of node 200, OADM 210 includes trunk ports 214 and 216, which are connected to optical path 110 for receiving and transmitting the WDM signals traversing the ring network 100. OADM 210 also includes local ports 2201, 2202, 2203, . . . 220m that serve as sources and sinks of traffic. Local ports 2201, 2202, 2203, . . . 220m are respectively connected to transponders 2301, 2302, 2303, . . . 230m. Each local port includes an add and a drop port so that each transponder serves as an access point to the ring network 100 for traffic to and from external users denoted by terminal equipment 2401, 2402, 2403, . . . 240m (e.g., Internet routers, LANS, and individual users). Signals between the transponders and the terminal equipment may be communicated in optical or electric form depending on the nature of the equipment.
FIG. 3 shows a functional block diagram of a network 300 consisting of two interconnected rings 310 and 320. Ring network 310 includes OADM nodes 312, 314, 316 and 318. Ring network 320 includes OADM nodes 322, 324, 326, and 328. The rings 310 and 320 are interconnected at a central office node 330, which incorporates OADM node 316 of ring 310 and OADM node 328 of ring 320. Central office node 330 also includes an optical cross-connect (OXC) 340 that communicates with OADM nodes 316 and 328. The OXC 340 is more flexible than an OADM and in some cases can redistribute the individual channel wavelengths onto any number of output paths. Unfortunately, regardless of whether the OXC core switch is optical or electrical, current OXC's generally employ optoelectronic regeneration at their network interfaces, thus requiring optical-to-electrical interfaces into and out of the cross-connect. The regeneration has historically been needed at such network interfaces because of propagation limitations of the optical signal due to loss, amplifier noise, chromatic dispersion, and or polarization mode dispersion. Therefore OXCs with optical switching in the core of the fabric still require regeneration, however as transmission methods improve to mitigate the aforementioned transmission limitations, it would be desirable to pass through an all optical OXC without OEO regeneration to avoid the extensive cost this entails. However, the current generation of OXCs have a relatively high insertion loss, which might still require regeneration, or at a minimum costly optical amplification of all incoming and/or outgoing signals. The high insertion loss arises from passing through three discrete components: wavelength demultiplexer, M×M switch, and then a wavelength multiplexer. In addition to the high insertion loss, such an arrangement gives rise to additional limitations, including the high cost of the components, and a lack of flexibility in routing the light between the input and output subsets of ports. Finally, the current generation of the OXCs has undesirable limitations as a separate network element since it requires space, and must be maintained and configured nominally independently of the elements it is connecting to. Accordingly, it would be desirable to develop a multi-wavelength optical network interface that provides optically transparent signal routing between rings or networks, thereby avoiding the need for a separate OXC network element, including an expensive switch fabric as well as OEO regeneration for the input and/or output of each and every wavelength interconnection in the network.